Acicular ferrite and bainite in C–Mn and low-alloy steel arc weld metals

Abstract

Acicular ferrite is recognised as a desirable microstructural constituent in C-Mn and low-alloy steel weld metals. It is Widmanstätten ferrite that nucleates on the spheroidal non-metallic inclusions and grows by a reconstructive (diffusion-controlled) mechanism that proceeds substantially to completion. With increasing alloy content and/or cooling rate, the transformation to acicular ferrite may not be completed before the formation of bainite begins, the efficacy of inclusions in modern weld metal being such that colonies of bainite can nucleate on inclusions, forming by a displacive (shear) transformation mechanism, and producing colonies of similar size to acicular ferrite laths. Hence, in arc welds deposited with modern welding consumables, both acicular ferrite and intragranular bainite may form. As a consequence of the similarity in appearance of these two microstructural constituents in the optical microscope, some confusion in terminology has been introduced into the literature. This review seeks to eradicate this confusion.

Efeito da Velocidade de Soldagem nas Características Geométricas e Microestruturais de Soldas por Arco Submerso em Tandem Arc

“…A bainita (B) e a ferrita acicular (AF) se formam por mecanismos de transformação similares, diferindo principalmente em seus sítios de nucleação. A ferrita acicular (AF) nucleia em inclusões presentes no interior de grãos austeníticos, enquanto a bainita nucleia em contornos entre grãos austeníticos ou entre grão ferrítico e grão austenítico [14,17,18,20]. Durante a solidificação enquanto houver contornos disponíveis, a bainita é favorecida cineticamente em relação à AF.…”

“…O resfriamento mais rápido e a permanência mais breve a altas temperaturas, que limita o crescimento de grão austenítico, favorecem a ocorrência de ferrita acicular e o refinamento da mesma. Esse microconstituinte parte de nucleação intragranular e apresenta crescimento adifusional radial em várias direções, o que leva a formação caótica de ripas cristalograficamente desorientadas entre si, com comprimento aproximado entre 5 e 15 μm e de 1 a 3 μm de espessura [9,[11][12][13][14][15]. A ausência de paralelismo entre as interfaces faz com que a configuração seja mais resistente à propagação de trincas, o que eleva a tenacidade do aço.…”

“…Taxas de resfriamento mais altas elevam o superresfriamento, que atua como força motriz da nucleação. Com maiores taxas de nucleação, tem-se menos espaço disponível ao crescimento das agulhas, o que resulta em arranjo fino de grãos intertravados [14,15].…”

Hydrogen Embrittlement Susceptibility of Gas Metal Arc Welded Joints from a High‐Strength Low‐Alloy Steel Grade S690QL

“…In the center of the dimples, inclusions can be found (Figure 11c, e), the formation of which is desired in high-strength steel WMs to serve as intragranular nucleation sites for acicular ferrite. [17][18][19][20][21] In the in situ hydrogen-charged condition, the longitudinal SSRT specimen from the WM of weld A exhibits a brittle fracture morphology (Figure 11b,d,f ). In the central area of the fracture surface (Figure 11d), rosettes appear as signs of quasicleavage fracture, whereby the local crack surface runs almost concentrically starting from a center and the individual fracture paths are intercepted in dimple fields.…”

Metallurgical Effect of Rare-Earth Lanthanum Fluoride and Boride in the Composite Coating of Wires in the Arc Welding of Bainitic-Martensitic and Austenitic Steel

“…REEs reduce sensitivity to HAC and increase the impact toughness of the weld metal due to microstructural refinement, the formation of acicular ferrite, and a high affinity for sulphur and oxygen [4][5][6]. Wang et al [6] performed arc welding of HSLA steel 10CrNi3MoV, with a 14 mm thickness, using flux-cored wire with a 0.3-1 wt% rare-earth concentrate of the total flux weight in the wire.…”